Britain has become the first country in the world to approve human trials that will deliberately expose healthy volunteers to COVID-19.  The research — which begins within a few weeks — will include 90 people, ages 18 to 30, who have agreed to be exposed to the smallest amount of the virus needed to cause infection. Researchers noted that the original SARS-CoV-2 virus circulating in England since March 2020 will be used, rather than new variants. The volunteers will be...